Provided are methods for coating plastic substrates with an aqueous coating composition comprising at least one organic polymer as binder, the coating composition being applied to the surface of the plastic substrate and being subsequently cured, wherein the plastic substrate is pretreated before the coating composition is applied, the pretreatment comprising the two following separate steps in the order stated: (A) heat-treating a plastic substrate at an ambient temperature in the range from 60° C. to 160° C. for a period in the range from 1 minute to 20 minutes, (B) flaming the surface of the plastic substrate pretreated as per step (A), there being a period of up to 90 minutes between the ending of step (A) and the beginning of step (B). Additionally, provided are coated plastic substrates that have been coated by the methods of the invention.